This invention relates to a bonding head for a wedge wire bonding apparatus, and in particular to such a bonding head having improved bonding force control and wire clamping control in the feeding and tearing mechanism.
In a wedge wire bonding apparatus, the bonding head comprises a number of sub-elements that provide functions essential to a successful wedge wire bonding process. Notably, the bonding head includes a transducer which provides the ultrasonic energy required for the bonding process, and the transducer in turn holds the wedge that directs the energy to the wire to form the bond. The bonding head is also required to apply a bonding force to press the wire with the wedge against a bonding pad on, for example, an electronic component (such as an IC die) to which the wire is being bonded. Naturally it is important that this bonding force be carefully controlled.
Another function of the bonding head is to break the wire after the bond has been formed and to feed further wire for the next bond. This is achieved by a wire clamping means that is positioned next to the wedge. To achieve the tearing and feeding of the wire, a mechanism is used to drive the wire along a feeding direction of the wire (and usually at 30xc2x0, 45xc2x0 or 60xc2x0 to the horizontal).
Conventionally, as described for example in U.S. Pat. Nos. 4,603,803 and 5,452,838, both the clamping force of the wire clamp and the bonding force are usually provided by solenoids. These conventional designs have a number of disadvantages, however. These disadvantages include reliability problems, the weight of the solenoids which makes a fast response often very difficult, and also the fact that accurate control of the bonding force and the wire clamp is insufficient for modern wire bonding requirements. Prior art designs also have poor linearity.
It is accordingly an object of the present invention to provide a wire bonding head that improves over the prior art in the performance of in particular the bonding force control and application, and in the control of the wire feeding and tearing mechanism.
According to the present invention therefore there is provided a bonding head for a wire bonding apparatus for bonding a wire to an electronic component comprising,
(a) a bonding member for applying a bonding force to a said wire, said bonding member being adapted to pivot about an axis,
(b) means for generating said bonding force comprising a permanent magnet motor for generating a force for causing said bonding member to pivot about said axis upon application of said force, and
(c) means separate from said bonding force generating means for moving said bonding head in a vertical direction towards and away from a said electronic component.
The use of a permanent magnet motor to generate the bonding force, provides a number of significant advantages over the prior art. In particular a permanent magnet motor permits reliable control of the bonding force that can be varied linearly over a wide position and current range. The permanent magnet motor is highly reliable and requires no parts that slide against each other causing wear.
In a preferred embodiment of the invention the apparatus is a wedge wire bonding apparatus and the bonding member comprises a wedge member fixed to a distal end of the transducer, and the permanent magnet motor generates a force causing the transducer to pivot about an axis whereby the bonding force is applied through the wedge member.
Preferably the transducer and the permanent magnet motor arc fixed to opposite sides of a transducer holder attached to a bond head bracket member by means of a flexure joint, whereby upon application of said force by the permanent magnet motor the transducer holder pivots about the flexure joint. In a particularly preferred arrangement the permanent magnet motor is a voice coil type device and a moving coil is disposed between a magnet or a plurality of permanent magnets fixed to the bond head bracket member. With the magnet(s) fixed to the bracket member and thus not forming part of the moving transducer assembly, a number of mechanical advantages are provided such as low mass of the moving transducer assembly and hence a faster response time. Preferably the permanent magnet motor, the longitudinal axis of said transducer and the direction of the applied bonding force are all disposed in a common plane.
The apparatus may also comprise a bonding force sensor, typically a piezoelectric force sensor or electrostrictive force sensor, which sensor may be used to provide a closed loop control to said voice coil. In addition programmable control means may be provided for enabling a preprogrammed bonding operation to be carried out. Additionally, a contact sensor may also be provided on the transducer holder for determining when the wedge member contacts the component.
The use of a permanent magnet motor in a wire bonding apparatus may also be useful in other parts of the apparatus, such as the wire feeding and tearing assembly. Viewed from another aspect therefore the present invention also provides a bonding head for a wedge wire bonding apparatus for bonding a wire to an electronic component comprising, wire feeding and tearing means for feeding a wire in a wire feeding and tearing direction and including wire clamping means for clamping a wire being fed and for breaking a wire after a bond has been formed, said wire clamping means comprising a pair of jaw members and means for causing opening and closing movement of said jaw members, wherein said opening and closing movement causing means comprises a permanent magnet motor formed of a coil located in a magnetic field created by one or more permanent magnets, wherein either of said coil and said magnet(s) is fixed to one of said jaw members for movement therewith in the wire feeding and tearing direction, the other of said voice coil and said pair of magnets being fixed to a bonding head supporting member.
In a preferred embodiment a first of the jaw members is fixed and the permanent magnet motor causes opening and closing movement of the other the jaw member. This movement may be caused by the coil being fixed to the movable jaw member and being located in a magnetic field produced by at least one magnet, such that the causes the movable jaw member to pivot relative to the fixed jaw member about an axis, which is preferably a preloaded bearing.
The wire clamping assembly is movable in the wire feeding direction, and to achieve this the jaw members and the moving coil are fixed to a bond head bracket member by means of a sliding unit, whereas the permanent magnet(s) are fixed directly to said bracket member. This means that the permanent magnet(s) do not move with the wire clamping assembly reducing the weight of the moving parts.
Using a permanent magnet motor in the wire feeding and tearing assembly provides many advantages that are similar to those described with reference to the permanent magnet motor being used to create the bonding force. These advantages include easy linear control of the wire clamping force and high reliability.
Viewed from a further aspect the present invention provides a bonding head for wedge wire bonding apparatus for bonding a wire to an electronic component comprising,
(a) a bonding member for applying a bonding force to a said wire, said bonding member being adapted to pivot about an axis,
(b) means for generating said bonding force comprising a permanent magnet motor for generating a force for causing said bonding member to pivot about said axis upon application of said force,
(c) means separate from said bonding force generating means for moving said bonding head in a vertical direction towards and away from a said electronic component, and
(d) wire feeding and tearing means for feeding a wire in a wire feeding and tearing direction and including wire clamping means for clamping a wire being fed and for breaking a wire after a bond has been formed, said wire clamping means comprising a pair of jaw members and means for causing opening and closing movement of said jaw members, wherein said opening and closing movement causing means comprises a permanent magnet motor formed of a coil located in a magnetic field created by at least one permanent magnet, wherein either of said coil and said magnet(s) is fixed to one of said jaw members for movement therewith in the wire feeding and tearing direction, the other of said voice coil and said pair of magnets being fixed to a bonding head support member.